This invention relates to a method of and apparatus for separating air.
The separation of air by rectification (i.e. fractional distillation) is very well known indeed. Typically, the air is introduced into a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler, of which the condensing passages communicate with an upper region of the higher pressure rectification column and the reboiling passages communicate with a lower region of the lower pressure rectification column. Nitrogen is thereby separated in the higher pressure rectification column and is condensed in the condenser-reboiler. A part of the resulting condensate is used as reflux in the higher pressure column and another part of the condensate is so used in the lower pressure rectification column. An oxygen-enriched liquid air fraction is taken from the bottom of the higher pressure rectification column and is introduced into an intermediate mass exchange region of the lower pressure rectification column. A nitrogen fraction is obtained at the top of the lower pressure rectification column and an oxygen-enriched fraction at its bottom. A nitrogen product is therefore obtained at the pressure of the lower pressure rectification column.
Many industrial processes, for example the enhanced recovery of oil or gas, require nitrogen to be supplied at an elevated pressure, often in excess of that at which the higher pressure rectification column operates. In order to reduce the amount of work required to raise the pressure of the nitrogen product from that of the lower pressure rectification column to that demanded by the process to which the nitrogen is to be supplied, it is known to take some of the nitrogen product as vapour from the higher pressure rectification column. For a given recovery of nitrogen, the rate at which such higher pressure nitrogen can be so taken is, however, limited because the withdrawal of the nitrogen vapour from the higher pressure rectification column reduces the amount of liquid nitrogen reflux that is formed.
It is the aim of the present invention to provide a method and apparatus capable of being operated so as to mitigate the above-mentioned problem.